Dunno about vocals but interestingly many of the instrumentalists are formally and perhaps classically trained to a high degree in their instruments in metal bands as compared to pop, punk, alternative rock genres. Don't get me wrong, occasionally you can find real talent in those other genres, but some metal bands just simply have ridiculous keyboard, drum, guitar, based talent all in one band. Talent one might only expect in classical music or jazz.

I prefer listening to female voices, such as Sharon den Adel in within temptation.

This. It's also worth pointing out that not all singers of metal NEED to have that gruff quality. Quite a few were also classically trained before they sang for metal bands. Off the top of my head, Tarja Turunen (ex-Nightwish), Charlotte Wessels (Delain), and Elize Ryd (Amaranthe) initially sang jazz and/or classical first.

One thing people don't realize is that that gruff metal screaming is often times very quiet in terms of actual volume and amplified. They look like they are in pain doing it because it's part of the act.

Having the ability to get loud and remain in control takes talent. And getting REALLY loud and/or death growling and whatnot takes practice, unless you wanna ruin your vocal chords.

But there are plenty of metal singers who really aren't that remarkable...they can do a lot of different things with their voice- clean singing, screams, death growls...but in the end the 'singing' just isn't that special.

I think Dustin Kensrue (Thrice) is officially my favorite vocalist. The evolution of his voice has been quite remarkable.

A lot of metal singers (m/f) are either classically trained or practice their voices a lot. If you try to sing like that and don't do it 'right' you'll not be able to sing for very long.
But the human voice is capable of much more than many people think. Just look at beatboxers, or a band like Van Canto (for those who don't know them but know Nightwish, listen to this with headphones on: http://www.youtube.com/watch?v=Qr7j-a9Kxc0)

Real good vocalists: Roy Khan and Matt Barlow are less known than ones you mentioned, yet better.

I actually like Mustaine. His voice fits Megadeth's music well. He does a good job on tracks like 'Addicted To Chaos', 'The Disintegrators' etc. His vocals on Tornado Of Souls, a great track by itself, are awful though.

What I like is when a singer can adjust his/her voice to suit what they sing about in such a way that you can sense the emotion back in their voice.
My Dying Bride, Sentenced, Within Temptation, Opeth, Edge of Sanity, Eluveitie, and lots more do that.

Where pop music tends to have quite 'simplistic' subjects and therefor lyrics (mostly about love or happiness) metal has always also looked at the darker side of society. Both the pain of losing someone and the harm inflicted on people by others. That is also a reason why most metal will never be mainstream as most people don't want to be reminded about life not being all that nice and simple.
Whether it was Slayer singing about Joseph Mengele (Angel of Death), Sentenced (Killing Me, Killing You) or My Dying Bride (For My Fallen Angel) singing about a loved one dying, or even a subject like incest about which many bands have sung songs, including Megadeth (Family Tree), Rammstein (Spiel Mit Mir and Tier). Dark subjects no one wants to think about, but they're part of humanity and won't go away by keeping quiet about it.

Metal has gotten a bad reputation among many religious people due to the few bands that liked shocking people and therefor sang about Satanism. But many metal bands sing about serious subjects and deserve to be recognized by more people.
To fight the darker side of humanity and the mental and physical pain people can have you need to be able to understand it, and to understand it you first need to be able to talk about it. That's one of the reasons I have high hopes for the growing metal scene in the Middle East. Whether it's the Arabic metal bands breaking local taboos with their music and lyrics, the people who make the festivals possible there (Desert Rock was allowed as metal festival in Dubai thanks to a local princess that liked 'louder' music) or bands like Orphaned Land from Israel who perform alongside Arab bands.